SCTP, possible bug in peer authentication key

[Michael Tüxen]

Hi Yann,

very good catch! You are right.

I have committed your patch to Randalls repository, so it will
show up in the FreeBSD sources soon (next time he syncs them)...

Best regards
Michael

On Jan 28, 2009, at 8:51 PM, Yann WANWANSCAPPEL wrote:

> Hi all,
>
> I think I found a bug in the SCTP authentication code, in
> sctp_load_addresses_from_init() in sctp_pcb.c
>
> keylen = sizeof(*p_random) + random_len + sizeof(*chunks) +  
> num_chunks +
> sizeof(*hmacs) + hmacs_len;
>
> The keylen calculation assumes the Chunk List Parameter (CHUNKS)
> vl-param was present in the received INIT packet, which can be false  
> if
> peer SCTP does not require any chunk to be authenticated (this  
> typically
> occurs if peer does not support ASCONF).
>
>> From RFC 4895, 6.1
>
> * An SCTP endpoint has a list of chunks it only accepts if they are
> * received in an authenticated way.  This list is included in the INIT
> * and INIT-ACK, and MAY be omitted if it is empty.  Since this list
> * does not change during the lifetime of the SCTP endpoint there is no
> * problem in case of INIT collision.
>
> This case is properly handled later in the build of the key
>
> 	/* append in the AUTH chunks */
> 	if (chunks != NULL) {
> 	.....
> 	}
>
> I think the calculated keylen should be something like this :
>
> keylen = sizeof(*p_random) + random_len + sizeof(*hmacs) + hmacs_len;
>
> if (chunks != NULL) {
>   keylen += sizeof(*chunks) + num_chunks
> }
>
> This problem results in authenticated packets sent from peer SCTP to  
> be
> discarded.
>
> The problem does not occurs if peer SCTP is modified to send an empty
> Chunk List Parameter, (eg num_chunks = 0 in the decoding).
>
> Br,
> Yann
>
>
>
>
>
>
>
>
>
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